There has been disclosed a thin DC brushless motor for use in a floppy disk operating system in, for example, Japanese Utility Model Laid-Open Publication (Jikkaihei) 7-3283.
One known DC brushless motor is shown in FIG. 5. The DC brushless motor generally indicated by reference numeral 10 comprises a board 12 which is made of magnetic material. The board 12 has a circular opening 14 in which an annular bearing-housing 16 is securely inserted. The bearing housing 16 holds a bearing ring 18 at its inner periphery, by which a shaft 20 is rotatably supported perpendicular to the board 12.
An annular stator core 22 is coaxially secured on the bearing housing 16 by a plurality of screws 24. The stator core 22 has a plurality of projections 26 extending outwardly radially, around each of which an exciting coil 28 is wound. The shaft 20 holds an annular hub 30 coaxially to which a cup-like rotor frame 32 is supported to enclose the stator core 22. The rotor frame 32 has an annular hub magnet 34 located coaxially on its upper surface and an annular rotor magnet 36 on its peripheral inner surface while leaving a small gap from the distal ends of the projections 26. The rotor frame 32 has a magnet 38 at its circumferential edge for detecting a rotational frequency of the motor.
In addition, the board 12 has a slot 40 extending along the rotor magnet 36 so that a magnetic flux from the rotor magnet 36 is concentrated on the stator core 22.
Japanese Utility Model Laid-Open Publication (Jikkaihei) 4-80248 discloses another thin DC brushless motor in which a plurality of radially extended ribs are provided on the rotor frame to increase the strength thereof.
In these thin DC brushless motors, although a surface-opposing structure is mainly employed in the motor to increase an output torque thereof, it is preferable to employ a radial-gap structure from an economical point of view. However, the radial-gap structure must to decrease the number of the layers of the stator core to reduce the thickness of the motor. Therefore, to keep the output torque of the motor at a certain level, it is necessary to provide a member opposing the rotor magnet with a portion for preventing the magnetic flux from flowing into the opposing member, which adversely decreases both productivity and strength of the motor.